The present invention relates to device for the vacuum-refining of metal to free it from harmful gaseous and solid impurities.
Present-art devices for the vacuum-refining of molten metal comprise a vacuum chamber wherein a vessel with molten metal is placed. Such devices suffer from a low yield since only a surface metal layer is subjected to the vacuum treatment. Due to high hydrostatic molten-metal pressure the formation of gas bubbles at a considerable depth in the metal accommodated in the vessel is thermodynamically unfeasible.
Moreover, in the above device the temperature of metal diminishes substantially during the vacuuming period which makes necessary the additional heating of the metal to a higher temperature in the melting units prior to vacuum-refining. Vacuum treatment of considerable volumes of metal amounting to 100 - 200 t requires the use of high-capacity vacuum pumps which are complicated both in operation and maintenance. All this adds considerably to the cost of the vacuum-refining of molten metal. Operation of vacuum chambers of a high capacity adapted for vacuum-refining of metal having a high melting point poses a number of problems associated with the sealing and servicing of such chambers.
It is known that the rate of degassing of molten metal under a constant vacuum depends on the thickness of a metal layer to be vacuum-treated. Therefore the devices for vacuum-degassing of molten metal in the course of its circulation have been developed to intensify the vacuuming process. In such devices a vacuum chamber is fitted with metal feed and discharge pipe lines immersed into a vessel with molten metal. To effect the vacuum degassing of molten metal rarefaction is created within the vacuum chamber, with the molten metal filling as a result both the metal feed and discharge pipe lines. Metal circulation through the vacuum chamber is provided by blowing an inert gas into the metal feed pipe line. The above arrangement ensures higher quality of metal due to intensification of its vacuum treatment, metal degassing requiring thereby less time.
The use of inert gas for metal circulation presents a number of difficulties. The inert gas blown into a metal pipe line enters the vacuum chamber augmenting the residual pressure therein. This requires the use of more powerful vacuum pumps, decreases the yield of the degassing process and the temperature of molten metal owing to heat losses with the inert gas, this entailing additional expenditures for metal re-heating or preheating to higher temperatures before vacuum treatment.
Known in the art is a device for vacuum-refining of molten metal, wherein an electromagnetic pump is employed for feeding metal from a vessel into a vacuum chamber, the pump being set up on a metal feed pipe line. The device does not use inert gas for conveying molten metal, the result being an increased metal degassing degree and a higher yield.
However, this device also suffers from considerable losses of heat of the molten metal since the electromagnetic linear pumps fail to provide their compensation and the metal must be heated to a considerable degree prior to vacuum-refining. Moreover, when metals with a lowspecific gravity are subjected to vacuuming, the height of a static metal column offsetting a pressure gradient between the residual pressure in the vacuum chamber and atmospheric pressure above the metal surface in the vessel reaches a considerable value (4.3 m for aluminum) demands the use of such devices having large overall dimensions. This presents additional difficulties as far as their sealing and heating are concerned. In addition, the pouring of the vacuum-refined metal into moulds is conducted under normal atmospheric conditions, so that the metal becomes gas-contaminated again, this resulting in lower quality of ingots being cast.
Also known is a device for vacuum-refining of molten metal, comprising two electromagnetic pumps of which one is mounted on a metal feed and the other on a metal discharge pipe line. Such pumps are adapted to effect metal circulation through a vacuum chamber and to compensate for the static metal column offsetting a pressure gradient between the atmospheric pressure above the metal surface in the vessel and a residual pressure within the vacuum chamber.
The above pumps allow decreasing the overall dimensions of the device for vacuum-refining of molten metal. However, the use of two electromagnetic pumps complicates the design of the device for vacuum-refining of molten metal and diminishes its reliability in operation. In this case heat losses of metal during vacuum treatment cannot be compensated for in view of a rather low heat capacity of linear electromagnetic pumps.
Because of an ever growing amount of metal being cast and more stringent requirements to its quality, a need has arisen in the provision of devices of the type described providing a higher yield and a high degree of metal degassing.
The now-existing arrangements fail to satisfy all these requirements simultaneously.
However, a constant strive for devising continuous casting techniques for the production of castings and ingots requires the use of arrangements providing vacuum-refining of metal combined with simultaneous pouring of molten metal into foundry molds excluding any contact whatever between the degassed metal and air.